1. Field of the Invention
This invention relates to an apparatus which accomplishes alignment and further, automatic alignment, of the mutual positions of two bodies such as a mask and a wafer.
2. Description of the Prior Art
In an apparatus such as a semiconductor printing apparatus wherein two or more kinds of masks are used to effect printing on wafers in succession, the order of micron or submicron is required of the alignment accuracy of the mask and wafer in view of the tendency of elements toward minuteness. Such work has heretofore been carried out by skilled operators, but much time and labor have been involved in such work and automation of the work has been desired and recently has been put into practical use.
Under such circumstances, the enhancement of the accuracy and efficiency in the automatic alignment for mask and wafer is an important theme.
An aligning apparatus is disclosed in U.S. Pat. No. 3,683,195. In this apparatus, a mask and wafer having alignment marks are disposed in a superposed relation and these alignment marks are scanned to detect the positional relation between the alignment marks, thereby detecting the positional relation between the mask and wafer. The alignment marks of the mask are of the double cross type configuration in which first and second bars which are in parallel relationship are intersected at an angle of 90.degree. by third and fourth bars which are also in parallel relationship, and the alignment marks of the wafer are of such configuration in which fifth and sixth bars intersect each other at an angle of 90.degree..
The scanning of these alignment marks is effected by a movable opening disposed at the enlarged image position. This movable opening is in the form of a slit to increase the level of the detection signal.
Since the movable opening is in the form of a slit, two slit-like openings which are in orthogonal relationship are necessary for detecting the intersecting bars. The positional relation between the marks in a certain direction is detected by one slit, and the positional relation between the marks in a direction perpendicular to said certain direction is detected by the other slit.
However this conventional optical apparatus has required the provision of a plurality of slits when the problem of the quantity of light is taken into consideration, and the provision of the plurality of slits has led to complication of the optical system.
Also, the illuminating light for detection illuminates the entire area of an object to be observed and the light from a part thereof is detected by a photodetector and thus, only part of the illuminating light contributes to the detection and the illuminating light has not been effectively utilized.
There is another example of the apparatus in which the object surface is scanned by a spot. This apparatus has suffered from the disadvantage that failure of alignment or aggravation of accuracy occurs when dust is present on the scanning line or when alignment marks fall off.